In recent years, several observations have challenged the concept that immunoglobulin structural gene variants are inherited in strict Mendelian fashion. Therefore, a re-evaluation or re-interpretation of existing and new concepts form the focus of our current studies. Because of the close phylogenetic relationship of present-day lagomorphs, we have included in our animal system the immunoglobulins of the hare, cottontail rabbit and pika. Thus, our over-all objectives are to study further rabbit immunoglobulin allotypes as probes for monitoring the genetics and evolution of immunoglobulins in general. Toward that end, we are investigating the structural basis of allotype specificity by peptide and amino acid sequence analyses.